Charging by Conduction
In the previous two sections of Lesson 2, the process of charging by
friction and charging by inductionwere described
and explained. In this section of Lesson 2, a third method of charging - charging by
conduction - will be discussed. As was the case for charging by friction and
charging by induction, the process of conduction will be described and
explained using numerous examples of electrostatic demonstrations and lab
experiments.
Charging by conduction involves the contact of a charged
object to a neutral object. Suppose that a positively charged aluminum plate is touched to a neutral metal sphere.
The neutral metal sphere becomes charged as the result of being contacted by
the charged aluminum plate. Or suppose that
a negatively charged metal sphere is touched to the top plate of a neutral needle
electroscope. The neutral electroscope becomes charged as
the result of being contacted by the metal sphere. And finally, suppose that an
uncharged physics student stands on an insulating platform and touches a
negatively charged Van de Graaff generator.
The neutral physics student becomes charged as the result of contact with the
Van de Graaff generator. Each of these
examples involves contact between a charged object and a neutral object. In
contrast to induction, where the charged object is brought near but never
contacted to the object being charged, conduction charging involves making the
physical connection of the charged object to the neutral object. Because
charging by conduction involves contact, it is often called charging by
contact.
Charging
by Conduction Using a Negatively Charged Object
To explain the process of charging by contact, we will first
consider the case of using a negatively charged metal sphere to charge a
neutral needle electroscope. Understanding the process demands that you
understand that like charges repel and have an intense desire to reduce their
repulsions by spreading about as far as possible. A negatively charged metal
sphere has an excess of electrons; those electrons find each other repulsive
and distance themselves from each other as far as possible. The perimeter the
sphere is the extreme to which they can go. If there was ever a conducting
pathway to a more spacious piece of real estate, one could be sure that the
electrons would be on that pathway to the greener
grass beyond. In human terms, electrons living in the same home despise each
other and are always seeking a home of their own or at least a home with more
rooms.
Given this understanding of electron-electron repulsions, it
is not difficult to predict what excess electrons on the metal sphere would be
inclined to do if the sphere were touched to the neutral electroscope. Once the
contact of the sphere to the electroscope is made, a countless number of excess
electrons from the sphere move onto the electroscope and spread about the
sphere-electroscope system. In general, the object that offers the most space
in which to "hang out" will be the object that houses the
greatest number of excess electrons. When the process of charging by conduction
is complete, the electroscope acquires an excess negative charge due to the
movement of electrons onto it from the metal sphere. The metal sphere is still
charged negatively, only it has less excess negative charge than it had prior
to the conduction charging process.
Charging
by Conduction Using a Positively Charged Object
The previous example of charging by conduction involved
touching a negatively charged object to a neutral object. Upon contact,
electrons moved from the negatively charged object onto the neutral object.
When finished, both objects were negatively charged. But what happens if a
positively charged object is touched to a neutral object? To investigate this
question, we consider the case of a positively charged aluminum plate
being used to charge a neutral metal sphere by the process of conduction.
The diagram below depicts the use of a positively
charged aluminum plate being touched to a
neutral metal sphere. A positively charged aluminum plate
has an excess of protons. When looked at from an electron perspective, a
positively charged aluminum plate has a
shortage of electrons. In human terms, we could say that each excess proton is
rather discontented. It is not satisfied until it has found a negatively
charged electron with which to co-habitate. However,
since a proton is tightly bound in the nucleus of an atom, it is incapable of
leaving an atom in search of that longed-for electron. It can however attract a
mobile electron towards itself. And if a conducting pathway is made between a
collection of electrons and an excess proton, one can be certain that there is
likely an electron that would be willing to take the pathway. So when the
positively charged aluminum plate is
touched to the neutral metal sphere, countless electrons on the metal sphere
migrate towards the aluminum plate. There
is a mass migration of electrons until the positive charge on the aluminum plate-metal sphere system becomes
redistributed. Having lost electrons to the positively charged aluminum plate, there is a shortage of electrons on
the sphere and an overall positive charge. The aluminum plate
is still charged positively; only it now has less excess positive charge than
it had before the charging process began.
The above explanation might raise a rather difficult
question: Why would an electron on the previously neutral metal sphere desire
to move off the metal sphere in the first place? The metal sphere is neutral;
every electron on it must be satisfied since there is a corresponding proton
present. What would possibly induce an electron to go through the effort of
migrating to a different territory in order to have what it already has?
The best means of answering this question requires an
understanding of the concept of electric potential. But since that concept does
not arise until the next unit of The Physics Classroom, a different approach to an answer will be taken. It ends up that
electrons and protons are not as independent and individualized as we might
think. From a human perspective, electrons and protons can't be thought of as
independent citizens in a free enterprise system of government. Electrons and
protons don't actually do what is best for themselves, but must be more
social-minded. They must act like citizens of a state where the rule of law is
to behave in a manner such that the overall repulsive affects within the
society at large are reduced and the overall attractive affects are maximized.
Electrons and protons will be motivated not by what is good for them, but
rather by what is good for the country. And in this
sense, a country's boundary extends to the perimeter of the conductor material
that an excess electron is within. And in this case, an electron in the metal
sphere is part of a country that extends beyond the sphere itself and includes
the entire aluminum plate. So by moving
from the metal sphere to the aluminum plate,
an electron is able to reduce the total amount of repulsive affects within that
country. It serves to spread the excess positive charge over a greater surface
area, thus reducing the total amount of repulsive forces between excess protons.
Law
of Conservation of Charge
In each of the other methods of charging discussed in Lesson
2 - charging by friction and charging by induction - the law
of conservation of charge was illustrated. The law of conservation of charge
states that charge is always conserved. When all objects involved are
considered prior to and after a given process, we notice that the total amount
of charge among the objects is the same before the process starts as it is
after the process ends. The same conservation law is observed during the
charging by conduction process. If a negatively charged metal sphere is used to
charge a neutral electroscope, the overall charge before the process begins is
the same as the overall charge when the process ends. So if before the charging
process begins, the metal sphere has 1000 units of negative charge and the
electroscope is neutral, the overall charge of the two objects in the system is -1000
units. Perhaps during the charging process, 600 units of negative charge moved
from the metal sphere to the electroscope. When the process is complete, the
electroscope would have 600 units of negative charge and the metal sphere would
have 400 units of negative charge (the original 1000 units minus the 600 units
it transferred to the electroscope). The overall charge of the two objects in
the system is still -1000 units. The overall charge before the process began is
the same as the overall charge when the process is completed. Charge is neither
created nor destroyed; it is simply transferred from one object to another
object in the form of electrons.
Conduction
Charging Requires a Conductor
In all the above examples, the charging by conduction process
involved the touching of two conductors. Does contact charging have to occur
through the contact of two conductors? Can an insulator conduct a charge to
another object upon touching? And can an insulator be charged by conduction? A
complete discussion of these questions can get messy and quite often leads to a splitting
of hairs over the definition of conduction and the distinction between conductors
and 
insulators. The belief is taken here that only a conductor can conduct
charge to another conductor. The process of noticeably charging an object by
contact involves the two contacting objects momentarily sharing the net excess
charge. The excess charge is simply given a larger area over which to spread in
order to reduce the total amount of repulsive forces between them. This process
demands that the objects be conductors in order for electrons to move about and
redistribute themselves. An insulator hinders such a movement of electrons
between touching objects and about the surfaces of the objects. This is
observed if an aluminum pie plate is placed
upon a charged foam plate. When the neutral aluminum plate
is placed upon the charged foam plate, the foam plate does not conduct its
charge to the aluminum. Despite the fact that
the two surfaces were in contact, charging by contact or conduction did not
occur. (Or at least whatever charge transfer might have occurred was not
noticeable by the customary means of using an electroscope, using a charge
testing bulb or testing for its repulsion with a like-charged object.)
Many might quickly suggest that they have used a charged
insulator to charge a neutral electroscope (or some other object) by contact.
In fact, a negatively charged plastic golf tube can used to charge an
electroscope. The plastic tube is touched to the top plate of the electroscope.
On most occasions, the plastic tube is even rubbed or rolled across the plate
of the electroscope? Wouldn't this be regarded as charging by conduction? No.
Not really. In this case, it is more than 
likely that the charging occurred by some process other than conduction.
There was not a sharing of charge between the plastic tube and the metal parts
of the electroscope. Of course, once some excess charge is acquired by the
electroscope, that excess charge distributes itself about the surface of the
electroscope. Yet the charge is not uniformly shared between the two objects.
The protons and electrons within both the plastic golf tube and the
electroscope are not acting together to share excess charge and reduce the
total amount of repulsive forces.
The charging of an electroscope by contact with a negatively
charged golf tube (or any charged insulating object) would best be described as charging by
lightning. Rather than being a process in which the two
objects act together to share the excess charge, the process could best be
described as the successful effort of electrons to burst through the space
(air) between objects. The presence of a negatively charged plastic tube is
capable of ionizing the air surrounding the tube and allowing excess electrons
on the plastic tube to be conducted through the air to the electroscope. This
transfer of charge can happen with or without touching. In fact, on a dry
winter day the process of charging the metal electroscope with the charged
insulator often occurs while the insulator is some distance away. The dry air
is more easily ionized and a greater quantity of electrons is capable of
bursting through the space between the two objects. On such occasions, a
crackling sound is often heard and a flash of light is seen if the room is
darkened. This phenomenon, occurring from several centimeters away,
certainly does not fit the description of contact charging.
A charged insulating object is certainly capable of
transferring its charge to another object. The result of the charge transfer
will be the same as the result of charging by conduction. Both objects will
have the same type of charge and the flow of electrons is in the same
direction. However, the process and the underlying explanations are
considerably different. In the case of charging an object with a charged
insulator, the contact is not essential. Contacting the object simply reduces
the spatial separation betweentouching atoms and allows
charge to arc and spark its way between objects. Rubbing or rolling the
insulating object across the conductor's surface facilitates the charging
process by bringing a greater number of atoms on the insulator in close proximity
to the conductor that is receiving the charge. The two materials do not make
any effort to share charge nor to act as a single object (with a uniform
electric potential) in an effort to reduce repulsive affects.
Is this distinction between charging by conduction and
charging by lightning a splitting of hairs? Perhaps. For certain, each process
involves a transfer of charge from one object to another object, yielding the same
result - two like-charged object. Yet, distinguishing between the two forms of
charging is more consistent with the customary view that insulators are not
conductors of charge. It also serves to explain why some insulators clearly do
not always transfer their charge upon contact. This phenomenon of charging by
lightning will be revisited in Lesson 4 during a discussion of electric fields and
lightning discharges.
Check Your Understanding
Use your understanding of charge to answer the following
questions. When finished, click the button to view the answers.
1. A neutral metal sphere is touched by a negatively charged
metal rod. As a result, the sphere will be ____ and the metal rod will be ____.
Select the two answers in their respective order.
a. positively
charged
b. negatively
charged
c. neutral
d. much more
massive
e. ... not enough information to tell
Answer: BB
This is a case
of charging by conduction. When a charged object is used to charge a neutral
object by conduction, the previously neutral object acquires the same type of
charge as the charged object. The charge object maintains the same type of
charge that it originally had. So in this case, both objects have a negative
charge.
2. A neutral metal sphere is touched by a negatively charged
metal rod. During the process, electrons are transferred from the _____ to the
_____ and the sphere acquires a _____ charge.
a. neutral sphere,
charged rod, negative
b. neutral sphere,
charged rod, positive
c. charged rod,
neutral sphere, negative
d. charged rod,
neutral sphere, positive
e. ... nonsense! None of these describe what occurs.
Answer: C
During charging
by conduction, both objects acquire the same type of charge. If a negative
object is used to charge a neutral object, then both objects become charged
negatively. In order for the neutral sphere to become negative, it must gain
electrons from the negatively charged rod.
3. A neutral metal sphere is touched by a positively charged
metal rod. During the process, protons are transferred from the _____ to the
_____ and the sphere acquires a _____ charge.
a. charged rod,
neutral sphere, negative
b. charged rod,
neutral sphere, positive
c. neutral sphere,
charged rod, negative
d. neutral sphere,
charged rod, positive
e. ... nonsense! None of these describe what occurs.
Answer: E
Protons are
never transferred in electrostatic activities. In this case, electrons are
transferred from the neutral object to the positively charged rod and the
sphere becomes charged positively.
4. A metal sphere is electrically neutral. It is touched by a
positively charged metal rod. As a result, the metal sphere becomes charged
positively. Which of the following occur during the process? List all that
apply.
a. The metal sphere gains some protons.,/p.
b. Electrons are transferred from the sphere to the rod.
c. The metal sphere loses electrons.
d. The overall charge of the system is conserved.
e. Protons are transferred from the rod to the sphere.
f. Positive electrons are moved between the two objects.
Answer: BCD
In electrostatic
activities, protons are never transferred (which rules out choices a and e). Electrons are not positively charged (ruling
out choice e). Choices B, C and D are all true and explain the essential nature
of the conduction charging process.